1. Field of the Invention
This invention relates to Coriolis effect mass flowmeters for measuring fluid flow in a tube or pipe. More particularly, the invention relates to a free standing driver that serves to vibrate the tube or pipe at a resonance frequency without being mechanically grounded.
2. Background Information
Coriolis-type flowmeters have long been utilized to conveniently measure flow of a fluid through a tube or pipe. Although many types of devices may be utilized to perform this function, Coriolis-type devices offer the advantage of providing an output directly proportional to mass flow. This aspect typically enables such devices to provide accurate flow information with reduced need to consider variables such as fluid pressure, temperature and density, etc. Moreover, advantageously, there are no obstacles in the path of the flowing fluid.
The theory underlying a Coriolis-type mass flowmeter and the advantages gained thereby are discussed in an article by K. O. Plache, "Coriolis/gyroscopic Flow Meter" in the March 1979 issue of Mechanical Engineering, pages 36 to 39.
A Coriolis force is generally associated with a continuously rotating system. For example, winds moving uniformly away from the North Pole along a line that appears straight to an observer in space, would appear to an Earthbound observer to curve Westward. This is commonly referred to as the Coriolis effect. Moreover, a person moving on a turntable or merry-go-round at what appears to be a constant linear speed radially outward on the surface thereof, actually speeds up in the tangential direction. The change in tangential velocity indicates that the person has been accelerated. This Coriolis acceleration of the person generates a force known as a Coriolis force in the plane of rotation perpendicular to the radial movement of the mass. The person will experience this Coriolis force as a lateral force applied from the opposite direction as the acceleration and must lean sideways to compensate for it in order to continue to move forward along the merry-go-round's radius. In vector terminology, the Coriolis force vector is proportional and opposite to the cross-product of the angular velocity vector (parallel to the rotational axis) and the velocity vector of the mass in the direction of its travel with respect to the axis of rotation (e.g. in the radial direction).
It is this Coriolis force or effect that has been applied to mass flow measurement. If a pipe is rotated about a pivot axis orthogonal to the pipe, each discrete portion of material flowing through the pipe is a radially traveling mass which experiences acceleration. The Coriolis reaction force shows up as a deflection or offset of the pipe in the direction of the Coriolis force vector in the plane of rotation.
Coriolis mass flowmeters fall into two categories: continuously rotating and oscillating. The principal functional difference between these two types is that the oscillating version, unlike the continuously rotating one, has periodically (usually sinusoidally) varying angular velocity which produces a continuously varying level of Coriolis force.
Many Coriolis flow meters are dependent on phase shift measurements of the oscillation or twisting of the flow tubes during fluid flow. In a phase shift type device, a driver is mounted to a medial portion of the flow tube between the inlet and outlet thereof. When there is no flow through the flowmeter, all points along the flow tube oscillate with identical phase. As fluid begins to flow, Coriolis accelerations of the fluid cause areas along the flow tube to have a different phases. The phase on the inlet side of the flow tube lags the driver, while the phase on the outlet side leads the driver. Sensors can be placed on the flow tube to produce sinusoidal signals representative of the motion of the flow tube. The phase difference between two sensor signals is proportional to the mass flow rate of fluid through the flow tube.
Advantageously, an oscillatory system may employ the bending resiliency of the pipe itself as a hinge or pivot point for oscillation and thus obviate separate rotary or flexible joints. A major difficulty in these systems however, is that the effect of the Coriolis force is relatively small compared not only to the drive force but also to extraneous vibrations. These flowmeters thus tend to rely on being adequately mechanically grounded at the flexure point of the oscillating conduit, the driver and sensors. Unfortunately, however, provision of such a ground tends to be difficult, complicated and concomitantly, relatively expensive. The use of double tubes that vibrate at equal frequencies and opposite phase tend to increase accuracy by reducing errors from outside vibrations. However, use of such double tubes tends to disadvantageously add complexity and cost relative to single tube devices. Moreover, any dependence on exterior support structures for mechanical grounding can itself introduce vibration, such as may be transmitted to the tubes from nearby machinery or other structures.
In an attempt to address this grounding difficulty, devices have been provided that do not require mechanical grounding. One type of device measures torsional oscillations of a flow tube to calculate mass flow and is disclosed in U.S. Pat. No. 4,756,197, entitled CORIOLIS-TYPE MASS FLOWMETER, issued to Herzl, ("Herzl") which is fully incorporated herein by reference. In this device, the flow tube is a loop supported on a stationary frame, with the driver mounted at the vertex of the loop. When fluid is not flowing, the loop vibrates between parallel planes on either side of its static plane. When fluid passes through the loop, it is subjected to Coriolis forces, causing the vibrating loop to torsionally oscillate in accordance with the mass flow rate of the fluid. The torsional oscillations are sensed by a pair of strain gauge transducers mounted in balanced relation on opposite legs of the loop, whereby the signals yielded by the transducers have a difference in magnitude therebetween that depends on the amplitude of the torsional oscillations.
Another type of device that appears to utilize an ungrounded driver is disclosed in U.S. Pat. No. 5,321,991, entitled CORIOLIS EFFECT MASS FLOWMETER, issued to Kalotay, ("Kalotay") which is also fully incorporated herein by reference. This device utilizes a straight flow tube with an ungrounded magnetostrictive driver mounted at or near an anti-node of the second harmonic mode of the natural frequency of the tube section. One aspect of this device relies on determining phase shift using two sensors in the manner described hereinabove. Alternatively, this device may utilize a single sensor mounted to the pipe section at the node point of the second harmonic mode of the natural frequency of the pipe section during zero flow. This sensor is adapted to measure the amplitude of displacement of the zero flow node point due to the Coriolis effect forces from the mass of the material flowing through the oscillating pipe. This measurement is indicative of the mass flow rate of the material flowing through the pipe.
Both Herzl and Kalotay, however, utilize drivers that have limitations. The Herzl driver is a magnetic device configuration that may be difficult to control in an accurate and repeatable manner. Kalotay's magnetostrictive driver may be easier to control accurately than the Herzl driver, but tends to be relatively complex, expensive, and is generally limited in the amount of vibrational force and amplitude it can apply to a flow tube. Moreover, this driver appears to rely on the inertia of a relatively large, stationary mass. This characteristic itself tends to make Kalotay's driver cumbersome and undesirable for many applications where compact size and light weight are preferred.
A need thus exists for a free-standing driver for a Coriolis flowmeter that is relatively easily controlled, can generate relatively high levels of force and amplitude in a repeatable manner and is comparatively inexpensive to manufacture.